We forget he's 23 and a German NT regular, has played and won a CL final for the club. It seems Arsenal, Bayern and Real feel that we haven't used him well making him play #9.
Raw and untested in a competitive environment. Even if he turns out to be as good as his potential, we can always bump off the keeper we sign now and make space for him - like Courtois replacing Cech. We should be targeting a top-tier GK regardless of Slonina.